Four stroke internal combustion engines having varying numbers of cylinders are commonly used in various types of vehicles as they are known to make relatively efficient use of fuel as opposed to a two stroke internal combustion engine. For certain applications however, it is desirable to make use of the improved torque characteristics associated with two stroke engines. Adapting a conventional four stroke engine however, into a conventional two stroke engine is generally a costly and time consuming procedure as adjustment of the relative orientation of the pistons as well as the location and timing of the valves comprises a substantial replacement of parts.
U.S. Pat. No. 5,154,141 to McWhorter describes an internal combustion engine which operates as a four stroke engine at low speeds and as a two stroke engine at higher speeds. The cycle frequency of the valve operation is doubled for two stroke operation by gearing the camshaft to rotate at crankshaft speed rather than at half crankshaft speed as in four stroke operation. In order to successfully convert from four stroke operation to two stroke operation the engine requires a complex arrangement of a gas ejector and an electronic timing circuit which controls the rate of fuel injection and spark ignition. The complex arrangement of numerous parts results in a costly and high maintenance engine design in order to make use of the benefits of two stroke operation.